1. Field of the Invention
The present invention relates to a method for extracting the margin line for the designing of an artificial crown, which is used for complementing the defective part of a defective tooth.
2. Related Background Art
Conventionally, an artificial crown has been prepared by a manual process called waxing-up method, as shown in FIGS. 6A to 6C. At first a plaster cast of the abutment tooth, as shown in FIG. 6A, is prepared. The dentist forms the abutment tooth by shaping the diseased part, and said plaster cast is prepared from an impression of said abutment tooth. Then a margin line 61 is drawn on said plaster cast of the abutment tooth. The position of said margin line is determined by the dental technician, who observes the plaster cast and attaches marks where he thinks the margin line exists. Subsequently, as shown in FIG. 6B, molten wax is heaped on the plaster cast of the abutment tooth in such a manner that the wax does not overflow the margin line to the side of the plaster cast of the abutment tooth and that the wax is smoothly shaped in the vicinity of the margin line. In this manner the shape of the artificial crown is prepared as shown in FIG. 6C. An impression is prepared by thus prepared model of the artificial crown, and the artificial crown is prepared by said impression.
In such conventional method, the determination of the margin line depends greatly on the skill of the technician. An inadequate margin between the abutment tooth and the artificial crown leads to the detachment of the artificial crown or to the dental caries inside the artificial crown. Also such inadequate margin requires additional adjustment by the dentist at the attachment of the artificial crown to the diseased part, thus necessitating a longer time of treatment.
The preparation of the artificial crown, which has been manually conducted up to now, will surely depend upon CAD/CAM (computer aided design/computer aided manufacturing) in the near future.